Add AutoDeploy::Tag class

Extracted for !1315 (comment 464392038).

Adds an AutoDeploy::Tag class that encapsulates logic for the name of auto-deploy tags, including the timestamp, and removes the AutoDeployBranch#tag_timestamp method.

If running in a tagged release-tools pipeline, this will simply return that tag. Otherwise we build the tag based on the current auto-deploy branch and the current time. The current time is defined as a constant at runtime so that, for the current uncoordinated pipeline, a single job tagging multiple projects will all have the same timestamp.